Our objective is to establish the potential value for medical practice and hematological research of a new technique using antibody coated bacteria to count different lymphocyte subpopulations directly on blood smears. The rationale is that the experimental conditions exist for obtaining more information from a blood smear, namely subdividing lymphocyte subpopulations which are morphologically alike but functionally very different. This may lead to a better understanding of the role played by lymphocyte subpopulations in various diseases. The variation in relative proportions and absolute number of T cells, B cells and "null" cells will be investigated. Our preliminary results showed that T cells and B cells could be simultaneously labelled and enumerated in rabbit blood smears by using differently shaped bacteria coated with antibody. Preliminary experiments showed that this model may be applied to human blood smears. Our main specific aims are: 1) to demonstrate that using proper reagents and optimized technical conditions, lymphocyte subpopulations could be accurately identified in human blood smears; 2) to demonstrate the potential value of identifying B and T cells in blood smears for diagnosing leukemias, pre-leukemia states or other diseases; 3) to investigate the potential of differentiation of lymphocyte like cells to megakaryocytes, and to establish whether such cells belong to a known or yet unknown subpopulation of lymphocytes; to investigate the physiologic factors which may produce changes in the ratio between T and B cells in the peripheral blood of experimental animals and man. Starting from this investigation a large benefit for medical practice may result. Since bacteria coated with purified antibody, as well as the other reagents used in these experiments are stable reagents which may be standardized, they may become accessible to any hospital or practicing physician.